A head-mounted display (HMD) system is a type of wearable device with increasing popularity within the consumer electronics industry. HMDs, along with similar devices such as helmet-mounted displays, smart glasses, and virtual reality headsets, allow users to wear a display device such that the hardware remains fixed to their heads regardless of the user's movement.
When combined with environmental sensors such as cameras, accelerometers, gyroscopes, compasses, and light meters, HMDs can provide users with experiences in virtual reality and augmented reality. Virtual reality (VR) allows a user to be completely submerged into a virtual world where everything the user sees comes from the display device. Devices that provide augmented reality (AR) allow users to optically see the environment, and images generated by the display device are added to the scene and may blend in with the environment. Accordingly, traditional VR and AR technology involves a display that is mounted in front of the user's head with a lens configuration that determines the virtual image position and field.
A basic layout of typical commercial VR or AR systems (both involving and not involving use of a smartphone for a display) includes a display device and a lens structure that images the display light into the far field to enable comfortable viewing. To ensure sufficient magnification, with wide field of view and to have a virtual image at a far enough distance from the eye, the size of this arrangement is restricted. In addition, the display is a relatively far distance from the eyes, meaning that the device must be strapped to the head to not fall off. Furthermore, the weight of the device is far forward when worn, meaning that long term viewing could become tiresome on the face and neck due to the torque generated about the head by the weight of the device. Lens elements used in such systems may be configured as a normal curved surface lens of known type, or a structured Fresnel lens with angled features of known type, or other known lens arrangements involving one or more lenses.
One of the primary elements of HMDs is the display module mounted onto the head. However, since the unaided human eye cannot accommodate (that is, change optical power to provide a focused image) for images closer than a certain distance from the eye, eyepiece lenses are required to re-image the display module such that the display image appears to be at a comfortable viewing distance from the user. Such optical configuration requires substantial space between the eyepiece and the display module. Furthermore, complex lenses are needed if the HMD needs to display images with high quality and a wide field of view (FOV). The result of these requirements in conventional systems is a heavy and bulky headset that is uncomfortable to wear for any length of time, and the size is limited by basic optics to achieve the correct magnification and the virtual image distance.
A number of known methods have been used to attempt to achieve image panel-based, light-weight HMDs without the need for bulky eyepiece lenses. “Light-field displays” (also known as integral imaging) provides for one of the thinnest types of HMD systems, but along with other magnifier-based HMDs, light-field displays suffer from a fundamental limit whereby the display's resolution is heavily traded off for a smaller hardware form factor.
One known method for reducing the size and weight of an HMD system includes the use of a polarization reflection approach to reduce size. Such configurations, however, suffer from ghost image formation. Another method uses multiple small lenses with overlapping images that modify the magnification required. However, such an arrangement has a lower apparent resolution and can suffer from visibility of the image overlap. Applicant's commonly owned Application GB 1621621.0, filed Dec. 19, 2016, describes an alternative overlap method with two displays and a folded W-shaped mirror arrangement. The field of view in this case is defined by the maximum aperture and path length.